1. Field of the Invention
The present invention relates to a light-emitting apparatus, particularly to such an apparatus constituted by light-emitting diodes or laser diodes.
2. Description of the Prior Art
Light emitting diodes and laser diodes are devices used as light sources in electronic photoprinters, image reading systems and optical communications devices.
These light-emitting devices are constituted as a junction of P and N type semiconductor material to form a light-emitting section which is made to emit light of a prescribed wavelength by applying a forwardly-biased voltage across the P-N junction.
FIG. 10 shows an arrangement in which this type of light-emitting device is used as the light source of an electronic photoprinter. In FIG. 10A a multiplicity of these light-emitting devices are arranged to form an array 10. In this arrangement the light is made to scan by sequentially switching the elements on and off, and the light thus emitted is converged onto a photosensitive member 14 by a rod-array lens 12. In the arrangement shown in FIG. 10B, a polygonal mirror 16 rotating at a prescribed speed is used to scan light emitted from a single light-emitting element 11. In the arrangement shown in FIG. 10C, the photosensitive member 14 is scanned by light from a single light-emitting element 11 which is deflected by means of a mechanical scanning system 18.
A problem with these light-emitting elements used as light sources in electronic photoprinters, image reading systems and optical communications devices is that they have a low energy efficiency, with most of the injected electrical energy being converted into heat in the light-emitting section. This heat raises the temperature of the light-emitting section, which shifts the center frequency of the emitted light towards the longer wavelength side of the spectrum and reduces the light emission intensity.
This decrease in light intensity means a decrease in the intensity of light impinging on the photosensitive member which, in the case of an electronic photoprinter, can degrade the image quality and make it impossible to reproduce tone densities. In the case of an image reader the decrease in light intensity can degrade read accuracy, and in optical communication systems in can produce a deterioration of the signal-to-noise ratio.